As the problem of electromagnetic interference on the interior and exterior of modules caused by the requirements of electronic products for lightness, thinness, shortness and smallness and the high frequency and high speed of a communication system is more and more serious, the electromagnetic shielding becomes inevitable.
Currently, the main anti-electromagnetic interference techniques comprise: the shielding technique, the grounding technique and the filter technique. A circuit board including a flexible circuit board, a rigid board and a rigid-flex circuit board realizes the anti-electromagnetic interference mainly in consideration of the following factors: printing conductive silver paste, adding a metal copper layer or laminating an electromagnetic shielding film. In consideration of flexibility and thickness, the electromagnetic shielding film has better operability and practicability. The electromagnetic shielding film is widely applied to the flexible circuit board and the rigid-flex circuit board.
The existing shielding film mainly has the following structures:
First Structure:
In Chinese Patent Application Publication Number CN101176388A titled “Shielding Film, Shielding Printed Circuit Board, Shielding Flexible Printed Circuit Board, Shielding Film Manufacturing Method and Shielding Printed Circuit Board Manufacturing Method”, a shielding film is disclosed. An outmost rigid layer and a flexible secondary outer layer form an insulation layer, a solid metal conductor layer is formed on the flexible layer, and a thermocured electrically conductive adhesive layer is formed on the solid metal conductor layer. Due to the arrangement of the solid metal shielding layer, the shielding film has a higher shielding effectiveness. However, as the frequency is increased, and more particularly when the frequency is over 1 GHz, the shielding effectiveness of the shielding film is largely reduced, and the requirement of the shielding effectiveness of 60 dB or more may not be met. Therefore, to manufacture products with the shielding effectiveness of 60 dB or more demanded, the manufacturers still selects to print a certain thickness of silver paste.
In Chinese Patent Application Publication Number CN101448362B titled “ultrathin shielding film capable of changing circuit impedance, circuit board and manufacturing methods of ultrathin shielding film and circuit board”, a three-layer structure of a product is disclosed. An insulation layer is the outmost layer, a metal conductor layer is the second layer, and a thermocured electrically conductive adhesive layer, which is coated onto the metal conductor layer, is the third layer. The key points of the patent application are: the final impedance control is realized by changing the grid size of the metal conductor layer, and meanwhile, the functions of a shielding film are also provided. But the shielding effectiveness of the product with the three-layer structure is equal to that of the above-mentioned shielding film, and the same shortage exists. In addition, in the above-mentioned two patent applications, the shielding function is realized by utilizing a structure that one metal conductor layer is provided and the electrically conductive adhesive layer is coated onto the metal conductor layer, so that the disclosed shielding films merely are general shielding films.
Second Structure:
In Chinese Patent Application Publication Number CN1842245A titled “Electronic Assembly with Conducting Layer, Electrically Conductive Adhesive Film and Manufacturing Method of Electrically Conductive Adhesive Film”, a two-layer structure is disclosed. The outmost layer is a metal conductor layer, and the second layer is an electrically conductive adhesive layer. Compared with the first structure, the second structure has the characteristic that the outmost layer does not have an insulation layer and may be directly connected with metal. The shielding structure is: one metal conductor layer is utilized, and then the electrically conductive adhesive layer is coated. Therefore, the shielding effectiveness of the utilized shielding structure is not of essential difference with that of the first structure.
Third Structure:
In Chinese Patent Application Publication Number CN101120627A titled “Electromagnetic-Wave-Shielding Adhesive Film, Manufacturing Method of Adhesive Film and Method for Shielding Adherent from Electromagnetic Wave”, a two-layer structure is disclosed. The outmost layer is an insulation layer, and the second layer is an omnibearing electrically conductive adhesive layer. Compared with the first and second structures, the third structure has the characteristics that the structure, without the solid metal film layer, may meet the requirement for a thinner product, and has better bending resistance and lower cost. However, the most important index of the shielding film is the shielding effectiveness, and the solid shielding metal conductor layer is not provided, so that the shielding effectiveness of the structure cannot be up to 40 dB when the transmission frequency is over 1 GHz.
Fourth Structure:
In Chinese Patent Application Publication Number CN2626193Y titled “Composite Material with High Thermal Conductivity and Electromagnetic Shielding Function”, a composite material with two shielding layers is disclosed. However, an electromagnetic shielding layer is in a shape of checkerboard, and a high-thermal-conductivity layer is arranged between the two shielding layers. On one hand, a thermally conductive adhesive contains ground-thermal-conduction particles, so that it cannot conduct charges accumulated in the shielding layers into a ground layer to realize the stable high-frequency signal shielding. On the other hand, a metal conductor layer is in the shape of checkerboard, which is not a solid metal shielding layer structure, so that the very high shielding effectiveness is not realized.
Fifth Structure:
In Chinese Patent Application Publication Number CN1819758A titled “Double Electromagnetic Shielding Film”, a double electromagnetic shielding film utilizing austenitic nickel-chromium type stainless steel as a sputtering target material is disclosed, wherein the double electromagnetic shielding film is formed by processing on a plastic surface. Furthermore, the whole production efficiency is very low, and the mass production of a rolled ultrathin shielding film may not be realized.
Sixth Structure:
In Chinese Patent Application Publication Number CN101521985A titled “Shielding Structure and Flexible Printed Circuit Board with the Shielding Structure”, a shielding film is disclosed. A one-side flexible copper-clad plate is formed in a coating manner, and then an electrically conductive adhesive is coated onto the metal layer, wherein the thickness of the metal copper layer is in the range of 1-6 microns, and the thickness of polyimide is in the range of 3-10 microns. The thickness of the metal layer is larger than 1 micron, so that it is far higher than the thickness of a metal shielding layer in the current market. The rigidity is largely increased, so that the flexibility requirement is not met. Meanwhile, a metal foil of the shielding film with the sixth structure needs a carrier support so that the technique is complex, the cost is high, and no market competitiveness exists.
Seventh Structure:
In Chinese Patent Application Publication Number CN101772996A titled “Shielding Film Applied to Printed Wiring Board and Printed Wiring Board”, a shielding film is disclosed. The shielding film applied to the printed wiring board and the printed wiring board, in which a metal layer is hard to break when being subjected to repeated sliding and bending from a large bending radius to a small bending radius, are obtained. A first metal layer is a layer formed by one or more scale-like metal particles, and the second metal layer is a porous layer with multiple holes. Therefore, the shielding film with the seventh structure is hard to realize ultrahigh shielding effectiveness.
The above-mentioned structures are: a structure of a complete metal conductor layer coated with the electrically conductive adhesive, or a structure of an omnibearing electrically conductive adhesive, or a thermally conductive structure with two grid metal conductor layers without a solid metal conductor layer and a grounding design. Therefore, according to the above-mentioned structures, the shielding effectiveness of 60 dB or more cannot be reached, or the bending performance cannot be met, or the peel strength cannot be met, or the anti-oxidization performance cannot be met, or the large-scale production of ultrathin rolled products cannot be realized.